Page 56 - 84 human physiolofy part-1

Chapter 4

Structure and function of neurons

Structure

Neurons are highly specialized for the processing and transmission of
cellular signals. Given the diversity of functions performed by neurons in
different parts of the nervous system, there is, as expected, a wide variety in
the shape, size, and electrochemical properties of neurons. For instance, the
soma of a neuron can vary in size from 4 to 100 micrometers in diameter.[1]

The soma (cell body) is the central part of the neuron. It contains the
nucleus of the cell, and therefore is where most protein synthesis occurs.
The nucleus ranges from 3 to 18 micrometers in diameter.[2] The dendrites
of a neuron are cellular extensions with many branches, and metaphorically
this overall shape and structure is referred to as a dendritic tree. This is Brain, brain stem, and spinal
where the majority of input to the neuron occurs. However, information chord.
outflow (i.e. from dendrites to other neurons) can also occur (except in
chemical synapse in which backflow of impulse is inhibited by the fact that axon do not possess
chemoreceptors and dendrites cannot secrete neurotransmitter chemical). This explains one way
conduction of nerve impulse. The axon is a finer, cable-like projection which can extend tens,
hundreds, or even tens of thousands of times the diameter of the soma in length. The axon carries nerve
signals away from the soma (and also carry some types of information back to it). Many neurons have
only one axon, but this axon may - and usually will - undergo extensive branching, enabling
communication with many target cells. The part of the axon where it emerges from the soma is called
the 'axon hillock'. Besides being an anatomical structure, the axon hillock is also the part of the neuron
that has the greatest density of voltage-dependent sodium channels. This makes it the most easily-
excited part of the neuron and the spike initiation zone for the axon: in neurological terms it has the
greatest hyperpolarized action potential threshold. While the axon and axon hillock are generally
involved in information outflow, this region can also receive input from other neurons as well. The
axon terminal is a specialized structure at the end of the axon that is used to release neurotransmitter
chemicals and communicate with target neurons. Although the canonical view of the neuron attributes
dedicated functions to its various anatomical components, dendrites and axons often act in ways
contrary to their so-called main function.

Axons and dendrites in the central nervous system are typically only about a micrometer thick,
while some in the peripheral nervous system are much thicker. The soma is usually about 10–25
micrometers in diameter and often is not much larger than the cell nucleus it contains. The longest axon
of a human motor neuron can be over a meter long, reaching from the base of the spine to the toes.
Sensory neurons have axons that run from the toes to the dorsal columns, over 1.5 meters in adults.
Giraffes have single axons several meters in length running along the entire length of their necks. Much
of what is known about axonal function comes from studying the squids giant axon, an ideal
experimental preparation because of its relatively immense size (0.5–1 millimeters thick, several
centimeters long).

Function

Sensory afferent neurons convey information from tissues and organs into the central nervous

56 | Human Physiology